1. Field of the Invention
This invention relates to a transfer-type electrothermographic recording method and a recording apparatus for use with the same.
2. Discussion of Background
The following electrothermographic recording methods have been conventionally proposed:
(a) A method as disclosed in Japanese Patent Publication 35-14722, using an electrothermographic recording medium comprising an electroconductive support and a resinous layer formed thereon, for which electric resistance decreases upon application of heat thereto. In this method, the resinous layer is electrostatically charged, and heat rays are applied to the electrostatically charged resinous layer to form a latent electrostatic image on the resinous layer, corresponding to an original image to be reproduced. The resinous layer comprises, for instance, polyvinyl chloride, polyethylene, polyester, polystyrene or a styrene - maleic acid copolymer. PA1 (b) A recording method as disclosed in Japanese Patent Publication 38-14347, in which an electrothermographic material made of, for instance, polyester, chlorinated polyvinyl chloride or vinyl chloride, which is sufficiently transparent to heat rays, is superimposed on an original image to be reproduced, electrostatically charged, and exposed to heat rays in such a fashion as to correspond to the original image to form a corresponding latent electrostatic image. The latent electrostatic image formed on the electrothermographic material is then reversely developed with a dry toner to form a visible toner image, and the toner image is fixed thereto.
In the above methods, an infrared ray is applied to the recording medium which is placed in close contact with an original image. Therefore, a large amount of energy is required for recording, and images with high resolution cannot be obtained. In addition, since these recording media are made of electrically chargeable materials, they are costly.
To eliminate the conventional shortcomings, a transfer-type electrothermographic recording method has been proposed. This method employs an electrothermographic recording medium which is constructed in such a manner that a resinous layer serving as an electrothermographic recording layer of which electrical resistivity is large at room temperature and is decreased when heated, is formed on an electroconductive layer. In this recording method, the resinous layer is uniformly charged, and a latent electrostatic image is formed on the charged resinous layer by applying thermal signals which correspond to an original image to be reproduced to the charged surface of the recording layer. The latent electrostatic image thus formed is developed with a toner of which polarity is the same as that of the latent electrostatic image to form a toner image. The thus formed toner image is transferred to a receiving medium, for example, a sheet of plain paper, and then fixed thereon.
The basic process of the above-mentioned conventional transfer-type electrothermographic recording method will now be described with reference to FIG. 1(a) to FIG. 1 (e).
In these figures, an electrothermographic recording medium 3 comprises an electrothermographic recording layer 1 and an electroconductive layer 2, on which the electrothermographic recording layer 1 is formed.
FIG. 1(a) is a schematic illustration showing a charging step, in which the electrothermographic recording layer 1 (hereinafter referred to as the recording layer 1) is uniformly charged. In this figure, the recording layer 1 is charged to a negative polarity by a negative corona charger 4. The means for charging the recording layer 1 is not limited to a corona charger, but, for instance, a roller charger and a brush charger can also be employed.
FIG. 1(b) is a schematic illustration showing a thermal-writing step, in which the thermal signals corresponding to an original image to be reproduced are applied to the recording layer 1 by using a thermal head 5. Thus, a latent electrostatic image is formed on the recording layer 1.
FIG. 1(c) is a schematic illustration showing a development step, in which the latent electrostatic image formed in the thermal writing step is developed by reversal development, using a toner 6 of which polarity is the same as that of the latent electrostatic image, so that a toner image is formed in the portion where the thermal signals were applied. In the development step shown in this figure, a negatively charged toner, which is hereinafter referred to as a negative toner, is employed.
FIG. 1 (d) is a schematic illustration showing an image transfer and fixing step, in which the toner image formed on the recording layer 1 is transferred to a receiving medium 7 such as a transfer sheet with application of positive charge thereto by a positive corona charger 8 for image transfer. The toner image transferred to the receiving medium 7 may be heated by application of heated air or by using a heat-application plate or roller for fixing the toner image onto the receiving medium 7. In this step, a transfer roller which applies positive charge (not shown) may be used instead of the positive corona charger 8.
FIG. 1(e) is a schematic illustration showing a cleaning step, in which the remaining toner and residual electric charge on the surface of the recording layer 1 are cleaned by a cleaning roller 9, after the image transfer and fixing step. The cleaning roller 9 serves as initialization means when the above electrothermographic recording medium 3 is used repeatedly.
By repeating the above recording process, digital information can be recorded even on a sheet of plain paper.
In the above-described conventional electrothermographic recording method, however, in the course of the recording process, corona charges, thermal signals for the formation of latent electrostatic images, and physical pressure for toner image transfer and fixing are repeatedly applied only to the front surface of the recording layer 1 where the toner image is developed. Therefore the surface of the electrothermographic recording layer physically deteriorates while in use. The result is that the reliability of this recording method is not high.